In medical radiography an image of a patient's tissue and bone structure is produced by exposing the patient to X-radiation and recording the pattern of penetrating X-radiation using a radiographic element containing at least one radiation sensitive silver halide emulsion layer coated on a transparent (usually blue tinted) support. The X-radiation can be directly recorded by the emulsion layer where only limited areas of exposure are required, as in dental imaging and the imaging of body extremities. However, a more efficient approach, which greatly reduces X-radiation exposures, is to employ an intensifying screen in combination with the radiographic element. The intensifying screen absorbs X-radiation and emits longer wavelength electromagnetic radiation which silver halide emulsions more readily absorb. Another technique for reducing patient exposure is to coat two silver halide emulsion layers on opposite sides of the film support to form a "double coated" radiographic element. Diagnostic needs can be satisfied at the lowest patient X-radiation exposure levels by employing a double coated radiographic element in combination with a pair of intensifying screens. The imagewise exposed and processed radiographic element is primarily intended for viewing by transmitted light. In a typical situation a medical radiologist studies the silver image with the radiographic element mounted on a light box, a white translucent illumination source.
Radiographic elements that contain tabular grain silver halide emulsion layers are described in the art and are known to provide advantages over radiographic elements that comprise layers of the more conventional spherical grain silver halide emulsions. See, for example, U.S. Pat. Nos. 4,994,355, issued Feb. 19, 1991; 5,021,327, issued Jun. 4, 1991 and 5,041,364, issued Aug. 20, 1991.
As illustrated by European Patent Application No. 0 430 115 A1, published Jun. 5, 1991, (hereinafter referred to simply as EP 0 430 115 A1), it is also known that radiographic elements containing tabular grain silver halide layers sometimes have lower silver covering power than is desired. EP 0 430 115 A1 describes increasing the covering power of a tabular grain silver halide emulsion in a radiographic element by adding to the emulsion a heterocyclic thione having the following formula: ##STR2## wherein Z represents sufficient carbon atoms to form a 5 membered or aromatic ring, or substituted 5 membered or aromatic ring, and R is an alkyl of 1-5 carbon atoms, a sulfoalkyl group of 2-5 carbon atoms, a dialkyl aminomethyl or a a hydroxymethyl group.
It is desirable to increase the covering power of radiographic elements containing tabular grain silver halide layers because this provides higher density for a given amount of silver or the same density from a lesser amount of developed silver. Increased silver density is desirable from the medical radiologist's point of view since a higher density image can provide more detail and aid in making a diagnosis. Furthermore, from a manufacturing and cost point of view, it is desirable to reduce the amount of silver that is necessary to coat a silver halide emulsion layer in a radiographic element.
U.S. Pat. Nos. 4,720,447 and 4,859,565 may, upon superficial examination, appear to be of some interest with respect to this invention since these patents describe the use of heterocyclic azole compounds as "density-and/or tone controlling compounds". However, these patents simply describe using such compounds in a photographic silver complex diffusion transfer reversal process (simply referred to as DTR process) wherein a silver image is formed in a non-radiation sensitive layer from a soluble silver salt. There is no suggestion that such compounds would have any effect in modifying the covering power of a silver image formed in the DTR process, much less a silver image formed from a tabular grain radiation sensitive silver halide emulsion of the type used in the present invention. Furthermore, each of the aforementioned patents teach and demonstrate that the heterocyclic azoles described therein do not substantially affect transmission densities (D.sub.TR) measured on silver images formed in the DTR process. See, for example, Tables 2 and 3 in each of the aforementioned patents. Accordingly, it is evident that U.S. Pat. Nos. 4,720,447 and 4,859,565 are not pertinent to the invention described herein which pertains to the use of a specific class of heterocyclic azoles to increase the covering power of a silver image formed from a radiographic element comprising a radiation sensitive tabular grain silver halide emulsion layer.
U.S. Pat. No. 4,728,601 describes the use of certain 2-alkylthio-4-hydroxy-1,3,3a,7-tetraazaindenes to modify the image tone in photographic elements and impart a netural tone to a developed silver image formed upon exposure and processing of the element. Such image toning materials have the following formula: ##STR3##
Wherein R.sub.1 is alkyl containing 6 to 11 carbon atoms or is a ring system and the groups R.sub.2 and R.sub.3 are each individually hydrogen or alkyl containing 1 to 4 carbon atoms.
There is no suggestion in U.S. Pat. No. 4,728,601 that the tetraazaindene compounds described therein have any effect upon the covering power of the silver halide emulsions described in the patent and, of course, contains no teachings with respect to radiation sensitive tabular silver halide emulsions. The patent is, however, of some interest with respect to the present invention since a number of the compounds described therein have been found to be effective to increase the covering power of silver images formed from radiation sensitive tabular silver halide emulsions according to this invention.
In light of the previous discussion, it is evident that it is very desirable to increase the covering power of developed silver formed from radiation sensitive tabular grain silver halide emulsions. Likewise, it would be desirable to have a silver image forming radiographic element comprising a radiation sensitive tabular grain silver halide emulsion layer that provides a silver image exhibiting increased covering power upon exposure and processing. This invention meets such desirable objectives.